The present invention relates generally to a method of measuring a vehicle's own speed by the Doppler radar principle.
In particular, the invention relates to a method of measuring a vehicle's own speed by the Doppler radar principle, in accordance with which microwaves are sent out at a transmission frequency by the vehicle. A portion of the microwave energy is reflected back to the vehicle at a receiving frequency. Upon reception at the vehicle, the received signal is mixed with a sample of the transmitted signal to produce a mixed signal, whereby Doppler frequencies are obtained. After pulse formation, the mixed signals are evaluated as Doppler pulse signals by a frequency measurement in the time range and in accordance with direction-of-travel, in order to form digital Doppler signal values.
A further aspect of the invention relates to a corresponding device for the measurement of the own speed of a vehicle having a Doppler radar sensor located on the vehicle. The device has a mixer which is fed with microwaves of the transmitted frequency and with microwaves received by the Doppler radar sensor. The device includes a pulse-former connected to an output of the mixer, a period measuring device which is connected behind the pulse-former and produces one digital Doppler frequency value for each digitalized period, and an evaluation device which is connected behind the period measuring device to obtain a mean value for the speed sought from a predetermined number of Doppler frequency values.
As is known, the measurement of a vehicle's own speed by the Doppler radar principle is based on the fact that a difference frequency occurs between the frequency of a wavefront transmitted by the vehicle--transmission signal--and the wavefront reflected by the ground, with respect to which the vehicle moves, in particular, horizontally, and received again by the vehicle. The difference frequency is proportional to the relative movement of the vehicle with respect to the ground. By the terms "Doppler signal" and "Doppler frequency" there is always to be understood in the present application the signal with the difference frequency.
For the technical practice out of the method, a radar module which comprises an oscillator--a Gunn oscillator--a circulator, a mixer, an antenna--horn radiator--and a directional coupler can be used. The oscillator produces the high-frequency signal oscillation which is fed to the antenna via the circulator. The antenna radiates this energy into space and, in particular, in the direction toward the ground over which the vehicle is travelling. The wave reflected on the ground is received again by the antenna, and the corresponding received signal is fed to the mixer via the circulator. A part of the transmission energy is furthermore fed to the mixer via the directional coupler, so that the received signal and the transmitted signal are superimposed on each other. A mixing process takes place in the mixer--mixer diode--which is acted on by the transmitted signal and the received signal, the mixing process forming, inter alia, the Doppler signal as difference frequency between the frequency of the transmitted wave and the frequency of the received reflected wave. The Doppler signal is made available via an amplifier for further processing.
In practical devices for measuring the own speed of a vehicle by the Doppler principle, there occurs as Doppler frequency not only a single difference frequency but a Doppler spectrum even upon measurement of the speed by means of only a single point of reflection. In the case of ground which is present in actual practice, such as asphalt surfaces or tilled land, which can be approximated by a random distribution of points of reflection of differenct reflection factors, the detectable Doppler signal is in addition disturbed by amplitude quenchings and phase jumps. Such a Doppler spectrum exhibits an irregular distribution of the amplitude as a function of the frequency, which is furthermore not constant with time. Variations can be noted, in particular, of the maximum on the frequency axis.
The vehicle speed determined in accordance with the above principle of measurement is thus subject to a large measurement error.
In order to reduce this measurement error, there is already known a Doppler radar speed indicator having a pulse-width discriminator which is intended to determine the maximum of the distribution curve of the Doppler signal or the Doppler frequency spectrum (West German Patent No. 32 19 819). That speed indicator comprises a Doppler radar sensor which is fastened on the vehicle, transmits microwaves, and receives a part of the waves reflected by the ground and mixes them with a part of the transmitted waves. The Doppler signals produced in this manner are amplified in an amplifying and pulse-forming device and a Doppler pulse signal is produced. The Doppler pulse signal is converted into a digital Doppler signal in a period counter. The output of the period counter serves as address for a storage for the evaluation of the digital Doppler signals. An adder increments by one the content of the storage under each controlled address for each Doppler pulse signal applied until a predetermined number of digital Doppler pulse signals has been completely incremented. Upon the occurrence of the nth Doppler pulse signal, the above-mentioned pulse-width discriminator determines the address assigned to the storage content n/2 as measure of the speed sought.
This known Doppler radar speed meter however has the disadvantage that, in order to obtain sufficient accuracy, the digital Doppler signals must be measured and evaluated over long periods of time if the number of n Doppler pulse signals is not to be too small and, thus the determination of the Doppler frequency (which is also known as center frequency) which corresponds to the speed of the vehicle, imprecise. The measurement and evaluation of the Doppler signals, however, takes place at the expense of the dynamic nature of the process and of the Doppler radar speed meter operating in accordance therewith. This deficient dynamic nature is particularly disturbing when it is to operate in combination with an anti-locking brake device.
In order to reduce the spectrum of the reflected radiation and thus decrease the integration time for the determination of a mean Doppler frequency which corresponds to the own speed of the land vehicle, a special antenna developed as slot antenna wave guide, is already known (West German patent No. 22 37 139). The slot antenna wave guide is aligned with its longitudinal axis parallel to the direction of the velocity vector and with its broad sides transverse thereto, parallel to the surface of the earth. It is provided, in addition, in its lower broad side with a longitudinal slot for the radar oscillations which extends somewhat outside the center of the broad side. In order to avoid a further distortion or broadening of the Doppler spectrum which can occur upon pitching motions of the vehicle, a two-channel Doppler radar device is known (West German Patent 22 37 139), in which, with two mixers, one Doppler signal is formed from the radiation in the direction of travel and one Doppler signal from the radiation opposite the direction of travel. The two signals are combined with each other upon the evaluation.
It is further known in this connection to compensate by a correction signal for an error in the Doppler signal due to vertical osciallations or changes in the distance of the vehicle from the ground. For this purpose an additional mixer having a diode is used which receives a quasi-optically mirrored radar radiation which is not subject to a Doppler shift but changes its phase position as a function of the distance of the antenna from the ground. The mixing of the so-called mirrored radiation with the residual signal of the transmitter remaining in the wave guide results in a low-frequency oscillation with which the error occurring upon the measurement of the Doppler signal can be compensated for. The above measures for increasing the accuracy of the measured Doppler frequency increase the expense and can limit the possibility of using the measurement principle, or they are only suitable to compensate for certain causes of distortion of the measured Doppler signals.
The object of the present invention is rapidly and accurately to determine a Doppler frequency which corresponds to the vehicle's own speed, even from irregular, strongly fluctuating Doppler spectra. No great additional expense is to be required for the corresponding evaluation of the Doppler signals.